resourceloader: Only conditional-wrap script responses with only=scripts

Follows-up 9272bc6c4721225. The shouldIncludeScripts method returns
true for only=scripts, but also for other responses (except for
only=styles, naturally).

Regular load.php responses that load both scripts and styles don't
need the conditional wrap because those requests should only be
made from the mw.loader client which inherently means the
environment has been provided already.

It's merely unnecessary and shouldn't have caused any issues since
it simply evaluates to true always. It was already correctly being
excluded from raw modules that run before the environment is
provided (such as startup, jquery and mediawiki).

Change-Id: I375a7a8039f9b3ad4909e76005725f7d975d8a5e